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LECTURE 18

Page history last edited by Paul Cooper 10 years, 1 month ago

Dr Cooper's comments: Is this all there is? Who did what?

 

 

Le Chateliers principle predicts that a system at equilibrium will tend to shift in the endothermic direction if the temperature is raised.  Conversely, an equilibrium can be expected to shift in the exothermic direction if the temperature is lowered.

 

Exothermic reactions: Increasing the temperature favors the reactants.

 

 

Endothermic reactions: Increasing the temperature favors the products.

Van't Hoff Equation is an expression for the slope of the plot of the equilibrium constant (lnK)as a function of temperature.

Formula  

 

The slope of the line gives the enthalpy of the reaction. 

 

For an exothermic reaction, Formula.The negative slope means that ln K, and therefore K itself, decreases as the temperature rises.

The subsequent decrease in K leads to a shift in the equilibrium away from the products. 

 

Formula

 

Formula

For an exothermic reaction, when the temperature is increased, Formula decreases and so the decreasing entropy of the surroundings has less importance, so there is less driving force for the forward reaction and the reactants are favored.

Formula

 

If we assume that Formula varies little with temperature over the temperature range of interest, then we can take it outside the integral.

 

 

For an endothermic reactionFormula  is positive and corresponds to the decrease of entropy in the surroundings. The overall driving force is the increase of entropy in the system.

 

Inorder to find the value of the equilibrium constant at a temperature T2 in terms of its value K1 at another temperature T1, it is necessary to integrate between these two points.

 

Formula

 

 

Comments (1)

Arifin Enam said

at 10:06 pm on Dec 11, 2009

Dr. Cooper, Stephen and I worked together on this. There were only 20 slides for this lecture.

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